Mesoscale Correlation Length Scales from NSCAT and Minimet Surface Wind Retrievals in the Labrador Sea

Abstract

Observations of the surface wind speed and direction in the Labrador Sea for the period October 1996‐May 1997 were obtained by the NASA scatterometer (NSCAT), and by 21 newly developed Minimet drifting buoys. Minimet wind speeds are inferred, hourly, from observations of acoustic pressure in the Wind-Speed Observation Through Ambient Noise (WOTAN) technology. Wind directions are inferred from a direction histogram, also accumulated hourly, as determined by the orientation of a wind vane attached to the surface floatation. Effective temporal averaging of acoustic pressure (20 min), and the interval over which the direction histogram is accumulated (160 s), are shown to be consistent with low-pass filtering to preserve mesoscale time- and spacescale signals in the surface wind. Minimet wind speed and direction retrievals in the Labrador Sea were calibrated with collocated NSCAT data. The NSCAT calibrations extend over the full field lifetimes of each Minimet (90 days on average). Wind speed variabilities of O(5 m s21) and wind direction variabilities of O(408) are evident on timescales of one to several hours in Minimet time series. Wind speed and direction rms differences versus spatial separation comparisons (from 0 to 400 km) for the NSCAT and Minimet records demonstrate similar rms differences in wind speed as a function of spatial separation, but O(208) larger rms differences in Minimet direction. These differences are consistent with spatial smoothing effects in the median filter step for wind direction retrievals within the NSCAT swath. Zonal and meridional surface wind components are constructed from the calibrated Minimet wind speed and direction dataset. Rms differences versus spatial separation for these components are used to estimate mesoscale spatial correlation length scales of 250 and 290 km in the zonal and meridional directions, respectively.